class Main {
    public static void main(String[] args) {
        String greet = "Hello! World";
        System.out.println("字符串是: " + greet);
        System.out.println("位置3处的字符: " + greet.charAt(3));
        System.out.println("位置7处的字符: " + greet.charAt(7));
                String example = new String("Hello! World");
                System.out.println("使用 subString(): " + example.substring(7));
                System.out.println("使用 toLowerCase(): " + example.toLowerCase());
                System.out.println("使用 toUpperCase(): " + example.toUpperCase());
                System.out.println("使用 replace(): " + example.replace('!', 'o'));
            }
        }
